There are many different applications in which it is necessary or desired to determine whether a fluid comprises a specific gas, i.e. a test gas, or not. In such applications it is common to perform sampling of gas molecules from the fluid by means of a gas probe and detect test gas molecules, if any, among the sampled gas molecules (i.e. gas sample molecules) by means of a test gas sensor. More specifically, gas sample molecules from the fluid are then allowed to enter into the interior of a gas probe housing through a gas probe orifice and test gas molecules, if any, among the gas sample molecules are detected by means of a test gas sensor arranged in gas communication with the interior of the gas probe housing. Furthermore, it might not only be necessary or desired to determine whether the test gas is present in the fluid or not, but also to measure the concentration thereof in the fluid. Then a measurement unit may be connected to the test gas sensor. The measurement unit interprets and measures signals from the sensor and provides, for example, a digital, electric, acoustic or optic signal corresponding to the concentration of the test gas molecules in a volume of gas sample molecules reaching the test gas sensor.
Leak testing is one application which may involve sampling of gas molecules from a fluid by means of a gas probe and detecting test gas molecules, if any, among the sampled gas molecules by means of a test gas sensor. In one leak testing approach, an object to be tested for leakage is pressurized with a tracer gas (the test gas is commonly denoted as tracer gas in leak testing), whereby tracer gas molecules pass through any leaks in the test object to the outside thereof. Gas molecules are then sampled external to the test object by means of a gas probe and tracer gas molecules, if any, among the sampled gas molecules are detected by means of a tracer gas sensor. In case tracer gas molecules are found among the sampled gas molecules, the test object comprises a leak. Furthermore, by measuring the tracer gas concentration in a volume of gas sample molecules reaching the tracer gas sensor by means of a measurement unit connected to the tracer gas sensor, the size of the leak may also be determined. Examples of commonly utilized tracer gases are helium, hydrogen, refrigerants, sulfur hexafluoride and carbon dioxide.
In another leak testing approach, the interior of a test object is evacuated and the tracer gas is sprayed onto the outside of the test object. Thereafter gas molecules are sampled from the interior of the test object by means of a gas probe and tracer gas molecules, if any, among the sampled gas molecules are detected by means of a tracer gas sensor.
The above described tracer gas methods may be applied for local leak testing, i.e. leak testing at a specific leakage testing point of the test object. When the above first described tracer gas method is applied for local leak testing, gas molecules are sampled at the specific leakage testing point external to the test object after pressurization of the test object with the tracer gas and tracer gas molecules, if any, among the sampled gas molecules are detected. When the other described tracer gas method is applied for local leak testing, the tracer gas is sprayed onto the test object at the specific leakage testing point. Thereafter gas molecules are sampled from the interior of the test object and tracer gas molecules, if any, among the sampled gas molecules are detected.
In addition, the above described tracer gas methods may be applied for so-called global leak testing, which also is called accumulation testing. In global leak testing, the test object is placed in a cabinet or test chamber, whereby it is tested whether the test object is leaking at any point or is leak tight, i.e. it is not tested whether there is a leak at a specific leakage testing point, but the “total” leakage of the test object is tested. When the above first described tracer gas method is applied for global leak testing, the test object is placed in a test chamber and pressurized with a tracer gas. Thereafter gas molecules are sampled from the volume in the test chamber outside the test object and tracer gas molecules, if any, among the sampled gas molecules are detected. When the other described tracer gas method is applied for global leak testing, the test object is placed in a test chamber and tracer gas is sprayed into the test chamber in order to surround the test object. Gas molecules are then sampled from the interior of the test object and tracer gas molecules, if any, among the sampled gas molecules are detected.
However, when a gas probe is utilized for sampling gas molecules from a fluid, different types of liquid and solid contaminants, such as e.g. dust, debris, oil and grease, may, together with the gas sample molecules, enter into the interior of the gas probe housing and further to a test gas sensor being arranged in gas communication with the interior of the gas probe housing. The test gas sensor and, thus, the measurements may be negatively affected by such contaminants. In addition, other components such as e.g. filters, hoses, tubes and valves arranged in gas communication with the interior of the gas probe housing may be negatively affected by such contaminants. For example, the life of such components may thereby be limited and the need for service and maintenance thereof may be increased. Thus, there is a need to protect the test gas sensor and any other components being arranged in gas communication with the interior of a probe housing from liquid and solid contaminants.
In addition, for different reasons it is also desired to be able to control the amount of test gas molecules reaching the test gas sensor. For example, some sensors have a limited measuring range in that they are saturated at high concentrations of test gas. Furthermore, some sensors are also harmed temporarily or permanently by high test gas concentrations. In addition, it is of course also desired to be able to switch off the sampling such that no test gas molecules reach the test gas sensor during certain periods.
WO 2005/001410 describes a leak detection system comprising a permeable member, which is arranged in gas communication with a tracer gas sensor. The permeable member is permeable to tracer gas (in this case helium) utilized in leak detection under specified conditions, but blocks other gases, liquids and particles. In one embodiment, the permeable member is made of quartz. The helium permeability of quartz varies with temperature, whereby the amount of tracer gas molecules reaching the sensor may be controlled by adjusting the temperature. At a relatively high temperature, helium permeability is high, whereas at a relatively low temperature, helium permeability is low, whereby sampling is switched off.
Thus, the permeable member disclosed in WO 2005/001410 could be arranged in a gas probe housing between the orifice and the test gas sensor in order to protect the test gas sensor and any other components being in gas communication with the interior of the gas probe housing from liquid and solid contaminants and to control the amount of test gas molecules reaching the test gas sensor (i.e. to protect the test gas sensor from saturation and over-exposure). However, a heating element is required in order to adjust the permeability of the permeable member, i.e. to achieve the control of the amount of test gas molecules reaching the sensor. In addition, the adjustment of the permeability of the permeable member is a relatively slow process, i.e. it takes a relatively long time to adjust the permeability and, thus, to adjust the amount of test gas molecules reaching the test gas sensor. Furthermore, the permeable member disclosed in WO 2005/001410 is mainly suited to be utilized when helium is the test gas and it is not suited to be utilized for applications on liquids, i.e. when gas molecules are to be sampled from a liquid in order to detect a specific test gas in the liquid.
Thus, there is still a need for an improved gas probe, which gas probe may be utilized for sampling gas molecules from a fluid, which gas probe comprises a probe housing with an orifice and which gas probe is adapted to be arranged such that the interior of the probe housing is arranged in gas communication with a test gas sensor, whereby the gas probe is improved such that a test gas sensor arranged in gas communication with the interior of the probe housing may be protected from liquid and solid contaminants as well as from over-exposure and saturation without the above mentioned drawbacks.